Method of producing dispersion strengthened metals



United States Patent 0 3,468,658 METHOD OF PRODUCING DISPERSION STRENGTHENED METALS Conrad D. Herald, Birmingham, and David M. Scruggs, Oak Park, Mich, assignors to The Bendix Corporation, a corporation of Delaware No Drawing. Filed Dec. 8, 1965, Ser. No. 512,461 Int. Cl. C22c 29/00 US. Cl. 75-422 7 Claims ABSTRACT OF THE DISCLOSURE A method for dispersing refractory particles in a metal having the steps of saturating the molten metal with anions of the refracting particles, adding the refracting particles, and agitating the molten metal.

This invention pertains to methods of producing dispersion strengthened metals by adding the dispersoid to the molten state of the metal thereby resulting in faster and more economical strengthening of the metals and furthermore extending the applications of dispersion strengthening.

In dispersion strengthening, a metal is formed with small particles of a refractory material evenly dispersed therein. These refractory materials may be oxides, nitrides, carbides, silicides, or borides and in these refractory materials the anions are respectively oxygen, nitrogen, carbon, silicon, and boron.

In prior methods, the refractory particles were homogeneously dispersed by several methods. In one method, the metal was made into a fine powder and the refractory particles were made into a fine powder, the powders were mixed thoroughly; and the mixed powders were compressed and then the powders were heated until they were sintered. This method was expensive and time consuming due to the necessity of making the metal into a powder.

Another method was to put a metal salt in an electrolytic bath, add the refractory particles, mix the particles in the plating bath, and then the metal with the refractory particles dispersed therein would plate out on the electrode of the bath. This method again was expensive and time consuming because of the low volume of metal that was plated on the electrode.

A further prior method was to mix two alloys one of which would contain the refractory particles and the other of which would contain the metal in which the particles were to be dispersed. With the proper mixtures and temperatures, a precipitate would form which would have the particles properly dispersed in the metal. This again was time consuming and expensive.

It has been desired by the art for many years to be able to add the refractory particles directly to the molten metal to achieve the proper dispersion with resulting strength and ductility improvements. This previously has not been possible since when the refractory particles were added directly to the molten metal, they would agglomerate and would not achieve the degree of homogeneous dispersion that was necessary for the desired metal characteristics. This invention teaches a method whereby the refractory particles may be added directly to the molten metal without agglomeration. This is accomplished by wetting the refractory particles completely with the metal, whereby they will not agglomerate, and the wetting is achieved by saturating the metal with the anion of the refractory particle while the refractory particles are being mixed in the molten metal.

There are many ways in which the molten metal can be saturated with the anion, a few of which will now be discussed. For illustrative purposes, an oxide refractory 3,468,658 Patented Sept. 23, 1969 particle will be assumed. The anion for an oxide particle is oxygen and the metal would then be saturated with oxygen throughout the mixing of the oxide refractory particle in the metal. Such saturation can be achieved by (1) supplying oxygen to the surface of the molten metal at a partial pressure which is equal to or greater than the equilibrium partial pressure at the solubility limit so that no oxygen can leave the molten metal, (2) adding feed stock to the molten metal which stock is high enough in the concentration of oxygen so that the melt is saturated with oxygen, (3) covering the melt with a liquid containing oxygen such as a molten salt of oxygen, (4) placing the melt in contact with sufiicient material to cause equilibrium to be reached and examples of this would be the use of an oxide crucible or the covering of the melt with oxide grain. These are examples of mannets in which the molten metal may be saturated with the anion.

The time at which saturation of the molten metal with the anion occurs may usually be determined by the time at which slag formation on the surface of the melt occurs. This slag formation should be kept to a minimum since it may interfere with the mixing process. The slag may be minimized by employing a reducing gas in the melt to deoxidize the melt, again assuming the anion is oxygen. The reduction should be controlled so that the melt is not excessively reduced and the proper saturation conditions are lost. Also, saturation may be determined by periodically sampling the melt and analyzing the sample for saturation. Slagging may also be controlled by selecting a refractory particle which has an anion which is slowly reactive to the metal.

As mentioned, the melt should be mixed in order to obtain the proper distribution of the dispersoid. Preferably the melt is mixed until the refractory particles are uniformly dispersed in the melt to provide a substantially homogeneous mixture. Such mixing may be achieved by the use of a high energy propeller, a mixing screw, or jets of inert gas. Mixing which produces cavitation has been found satisfactory to adequately disperse the refractory particles.

Satisfactory results have been obtained when the refractory particles are in the size range between 100 angstrom units and one micron. In this range, sintering and agglomerating of the refractory particies have been prevented.

Table I, which follows, illustrates preferred refractory particles for each of 17 different basis metals. The basis metals may be pure or alloyed.

TABLE I Refractory particles Basis metal Oxide Carbide Nitride Tin NNNNN Niobium Zirconium MNNNNNN X-Indicatcs preferred combinations A more complete list of refractory particles which may be used in this invention is as follows:

Oxides of aluminum, silicon, titanium, zirconium, thorium, uranium, yttrium, magnesium, beryllium, rare earth oxides, and mixed oxides such as zirconates and spinels.

Carbides of titanium, zirconium, chromium, tantalum, tungsten, molybdenum, and silicon.

Nitrides of boron, silicon, hafnium, titanium, aluminum, and zirconium.

Silicides of molybdenum, yttrium, tantalum and zirconium.

Borides of chromium, zirconium, titanium, and tantalum.

In Table I, combinations that are indicated by an X are preferred for optimum strength and ductility. However, certain other combinations may be used for special purposes. As examples, several metals such as nickel, gold, and platinum dissolve a fair quantity of carbon in the melt which is later precipitated out as graphite. This normally adversely affects fabricability and strength so that these combinations do not exhibit the improved characteristics normally possible with this invention. However, these materials with the large quantity of graphite dissolved therein may prove useful as bearing metals or other special uses.

As will be appreciated by one skilled in the art, the more refractory and stable particles should be chosen for the higher melting metals and alloys.

Two specific examples of this invention will now be described. For the pure metal, 0.2 percent yield strengths are shown in the property tables of the following examples.

Example I Temperature, F Room 300 400 500 Lead, Example I p s i 6, 000 4, 500 2, 500 2, 500 Pure lead, p.s.i 1, 200 0 O The p.s.i. figures are yield strength in compression.

Example II A hundred gram sample of tin with eight percent alumina of particle size 0.02 to 0.04 micron was melted and mixed with a propeller mixer in a Teflon crucible in an oxygen environment. The tin was melted and stirred until oxidation began to occur, and after a short period of time, the alumina was wet by the tin while being thoroughly mixed. The mixture was then briquetted, rolled, and cut into samples for yield strength testing in compression.

PROPERTY TABLE FOR EXAMPLE II Temperature, F Room 200 300 450 Tin, Example II, p s i 9, 500 7, 200 6, 500 3, 000 Pure tin, p.s.i 2,100 0 0 0 The p.s.i. figures are yield strength in compression.

Having thus described our invention, we claim:

1. The method of forming dispersion strengthened metals by dispersing refractory particles therein comprising the steps of heating the metal until it is in a molten condition,

substantially saturating the metal with the anion of the refractory particle to be dispersed therein, adding the refractory particles to be dispersed in the molten metal,

agitating the molten metal to sufficiently disperse the particles therein to obtain the desired characteristics.

2. The method of forming dispersion strengthened metals by dispersing refractory particles therein comprising the steps of heating the metal until it is in a molten condition,

substantially saturating the metal with the anion of the refractory particle to be dispersed therein,

adding the refractory particles to the molten metal,

agitating the molten metal to achieve substantially homogeneous dispersion of the particles therein. 3. A method of forming dispersion strengthened metals by dispersing refractory particles therein comprising the steps of heating a metal having a component therein selected from a group consisting of tin, lead, zinc, aluminum, copper, magnesium, cobalt, nickel, iron, chromium, indium, silver, gold, platinum, molybdenum, niobium, and zirconium until it is in a molten condition,

substantially saturating the metal with an anion from the group consisting of oxygen, carbon, nitrogen, silicon, and bon'de, of the refractory particle to be dispersed therein,

adding the refractory particles to the molten metal,

agitating the molten metal to obtain substantially homogeneous dispersion of the refractory particles.

4. The method of claim 1 having the step of preparing the refractory particles so that they have a diameter between angstroms units and one micron.

5. The method of claim 1 wherein the step of saturating the metal with the anion of the refractory particle to be dispersed therein comprises supplying a partial pressure of the anion above the melt which is equal to or greater than the equilibrium partial pressure at the solubility limit of the anion in the metal.

6. The method of claim 1 wherein the step of saturating the metal with the anion of the refractory particle to be dispersed therein comprises adding to the molten metal sufiicient quantities of metal stock having the anion therein to obtain saturation.

7. The method of claim 1 wherein the step of saturating the metal with the anion of the refractory particle to be dispersed comprises placing the molten metal in contact with material having the anion therein.

References Cited UNITED STATES PATENTS 2,580,171 12/1951 Hagglund et al. 75-122 2,793,949 5/1957 Imich 75-135 2,949,358 8/1960 Alexander et al. 75-135 3,028,234 4/1962 Alexander et al. 75-135 3,180,727 4/ 1965 Alexander et al. 75l35 RICHARD O. DEAN, Primary Examiner U.S. Cl. X.R. 

